In the coating of different kinds of substrates, it is known to employ the application of a liquid layer onto the substrate surface. Herein, the substrate surface is covered with a liquid film that can be postprocessed in order to dry and cure the same. On such branch of conventional technology employing the application of a liquid layer on a substrate surface is manufacture of glass.
A glass substrate with an antireflection coating on its surface is used particularly in photovoltaic devices to improve the efficiency of solar cells. By virtue of reducing the reflection of solar radiation from the surface of the device, a larger portion of the solar energy can be passed to the active area of the solar cell, thus allowing a higher efficiency in converting solar energy into electrical energy. The antireflection coating must be as thin as possible to minimize absorption of solar energy therein. Furthermore, the refractive index of the antireflective material must be substantially smaller than the refractive index of the glass used as a substrate. The refractive index of glass is typically about 1.5.
Antireflection coatings cause interference between the wavefronts reflected from the bottom and top surfaces of the coating. When the reflected waves are suitably out-of-phase with each other, they tend to interfere subtractively thus substantially reducing the amount of reflected light. Optimal efficiency of antireflectivity is attained when the refractive index is adjusted to match that of the substrate glass material and, additionally, the thickness of the coating is set to one-quarter of the wavelength for which the antireflection coating intended. However, as solar radiation comprises a relatively wide band of wavelengths, conventionally a multilayer antireflection coating is employed to achieve maximal transmission efficiency. As to terms of practical embodiments with regard to production speed and costs, however, the most advantageous result is attained with a single-layer coating featuring a gradually changing refractive index from the glass surface to air. This arrangement minimizes reflection even for varying angles of incidence of solar radiation onto the surface of the solar cell. Such a situation occurs, e.g., when solar radiation is incident on a permanently fixed solar cell at different times of a day.
It is known in the art to coat a substrate by a coating material produced from a liquid precursor that after the application of the coating is cured into a solid state. Painting a substrate, for instance, fulfills the specifications of such a treatment. In applications that require a minimal thickness, advantageously less than 1 μm, and a high surface smoothness from the coating, conventional coating methods fail to provide a satisfactory result. Moreover, antireflective surfaces pose extreme demands with respect to the surface smoothness of the coating.
In patent publication U.S. Pat. No. 4,871,105, American Telephone and Telegraph Company, AT&T Bell Laboratories, Mar. 10, 1989, is described a method and an apparatus for applying a flux flow at the surface of a substrate. The apparatus comprises means for converting a liquid flow into a fog flow and then injecting the fog flow into a laminar gas flow, said laminar gas flow being directed at the bottom surface of the substrate being processed, whereby a portion of the flux droplet flow adheres to the substrate surface thus coating the substrate. The patent publication does not mention the size of fog flow droplets. However, it can be deduced from the components (Sono-Tek piezoelectric crystal, model 8700) used in the most preferred embodiment that the diameter of the fog droplet is greater than 10 μm. Hence, producing smooth, thin coatings applied by virtue of this kind of large-droplet fog flow is difficult.
As discussed above, a problem in prior-art embodiments is that conventional arrangements fail to produce a sufficiently thin and smooth liquid layer onto the surface of a substrate. A thick and uneven liquid layer results in an uneven coating. Moreover, a thick and uneven coating fails to give a maximally good antireflective coating.